Water Clarity

What is Water Clarity?

Water Clarity is the measurement of how much sunlight can penetrate through the water. Water Clarity is measured as a distance in decimeter units (dm) from the water’s surface to the depth where sunlight can no longer penetrate the water. Water Clarity is diminished when solid particles, such as dirt and algae, are suspended in the water column and absorb or reflect sunlight.

Why is Water Clarity important?

High water clarity is important for the survival of aquatic plants and animals. High water clarity allows sunlight to reach aquatic plants, permitting critical fish habitat-forming plants like bay grasses to photosynthesize. High water clarity also indicates low levels of suspended solid particles, like sediment (silt and clay particles from soil) and algae, that may otherwise interfere with the ability of aquatic animals to respire (breathe) and locate food and habitat.

How do we measure Water Clarity?

Water Clarity is measured using an instrument called a Secchi Disk. The black- and white-colored weighted disk is attached to a graduated line and is lowered into the water up until the point that the disk is no longer visible. At the depth the disk disappears a reading is recorded on the line, and the Secchi disk is slowly retrieved until it reappears, at which point another reading is recorded. The average value of the two readings provides the Secchi depth data value. We assess Water Clarity data using the State of Maryland’s instantaneous numeric threshold of ≥10 dm for the protection of the ecological health of aquatic plant and animal populations in the Chesapeake Bay.